FIG. 1 is a diagrammatic longitudinal sectional view of part of a reheated or afterburning gas turbine engine 10 above the turbine rotational axis X-X. The gas turbine engine 10 includes a low pressure compressor 12, a high pressure compressor 14, a combustion system 16, a high pressure turbine 18 and a low pressure turbine 20. The combustion system 16 can operate on the reheat or afterburning principle and includes a primary combustor 22 and a reheat combustor 24 located downstream of the primary combustor 22. Both the primary and reheat combustors 22, 24 are annular and extend circumferentially around the turbine axis. The fuel burnt in the combustors can be, for example, oil, or a gas such as natural gas or methane.
In operation, air entering the gas turbine engine 10 is compressed initially by the low pressure compressor 12 and then by the high pressure compressor 14 before the compressed air is delivered to the primary combustor 22. Fuel is injected into the primary combustor 22 by a suitable fuel injector or lance 26, where it mixes with the compressed air. Alternatively, the fuel and air may be at least partially premixed together before the fuel/air mixture is injected into the combustion chamber. A plurality of circumferentially spaced burners 28 then ignite the fuel/air mixture to create hot combustion gases, which are expanded through, and thereby drive, the high pressure turbine 18.
Referring to FIG. 2, which shows a configuration of a known reheat combustor 24 in more detail, the expanded combustion gases are delivered through high pressure turbine outlet guide vanes (HP OGV's) 27 and vortex generators 29 to the reheat combustor 24 for reheating. The reheated combustion gases are directed through low pressure turbine inlet guide vanes (LP IGV's) 35 into the low pressure turbine 20 and exhausted from the engine. Both the high pressure and low pressure turbines 18, 20 are drivingly connected, via suitable connecting shafts, respectively to the high pressure and low pressure compressors 14, 12 which are, thus, driven in a known manner by the high pressure and low pressure turbines 18, 20.
The temperature of the hot combustion gases produced by the primary combustor 22 decreases as those hot combustion gases are expanded through the high pressure turbine 18. Because the power output of a gas turbine engine can be, proportional to the temperature of the combustion gases, it is desirable to reheat the combustion gases that have been expanded through the single-stage high pressure turbine 18 before they are expanded further through the multi-stage low pressure turbine 20. Although a single-stage HP turbine has been described, an HP turbine can have two or more stages if the combustion gases generated by the primary combustor have sufficient energy.
Referring again to FIG. 2, the reheat combustor 24 includes a fuel/gas mixer 30, which can be substantially annular but is segmented into a number of discrete mixing zones 25. The area referenced as 30 is not a continuous annulus but can include individual mixing zones 25 whose circumferential extents are defined by angularly spaced-apart side walls. However, the walls 44, 46, which define the radially inner and outer boundaries of the fuel/gas mixer 30, can be circumferentially continuous, though this is not essential. Each mixing zone 25 has an upstream inlet end 41 to receive the combustion gases 43 that have been expanded through the high pressure turbine and its annular array of outlet guide vanes 27. At the inlet ends 41, the combustion gases 43 pass through vortex generators 29 before fuel is injected into them by a fuel injector 32. The vortex generators 29 aid mixing of the injected fuel with the combustion gases 43 in the fuel/gas mixer 30. The mixture is delivered into an annular combustion chamber 34 through outlets 45 of the mixing zones and the mixture can spontaneously combust due to the heat of the combustion gases.
The number and spacing of the fuel injectors employed should be sufficient to ensure that the circumferential distribution of fuel, air and combustion gases around the mixing zones 25 is sufficiently uniform to enable adequate mixing before combustion occurs. It is desirable if there is one fuel injector per mixing zone of the fuel/gas mixer 30 but this is not an essential characteristic of the fuel/air mixer 30. For example, if each mixing zone has a sufficient circumferential extent, a more even distribution of fuel can be obtained if there are two or more fuel injectors per mixing zone. Assuming one fuel injector per mixing zone, it has been found that a suitable number of fuel injectors and mixing zones in a large heavy duty gas turbine engine can be twenty-four.
As the flame temperature in the reheat combustor 24 increases, the cooling requirements of the walls of the combustion chamber 34 and the fuel/gas mixer 30 can increase, as do the cooling requirements of the HP OGV's 27 and the LP IGV's 35 (FIG. 1). At the same time, the level of undesirable NOx emissions and the danger of premature ignition of the fuel/oxidant mixture can also increase. Hence, to control the level of NOx emissions and generally ensure efficient and reliable operation of the reheat combustor 24, it is desirable to provide suitable cooling for the reheat combustor 24 and associated components.
The HP OGV's 27 and the LP IGV's 35 can be cooled by convective and/or effusion and/or film cooling techniques, the cooling air being supplied from different sources, usually the high pressure and low pressure compressors, respectively. The annular combustion chamber 34 of the known reheat combustor 24 has walls including radially inner and radially outer annular double-walled combustion liners 40, 42, respectively, which can be convectively cooled by a supply of cooling air, which can be drawn from the low pressure compressor 12. The cooling air flows through radially inner and outer cooling paths 36, 38 defined between the double walls of the radially inner and radially outer combustion liners 40, 42. In contrast, the walls of the fuel/gas mixer 30 can be effusion-cooled. Specifically, radially inner and radially outer walls 44, 46 of the fuel/gas mixer 30 both can include a large number of holes having a small diameter (for example, about 0.7 to 0.8 mm) through which cooling air 47 effuses. Furthermore, the dividing walls between adjacent mixing zones 25 of the fuel/gas mixer can also be effusion cooled. The air for effusion cooling can be supplied from the combustion liner flow paths 36, 38, which exhaust into annular plenum chambers adjacent the radially inner and outer fuel/gas mixer walls 44, 46. Due to the acute inclination of the holes relative to the interior surfaces of the radially inner and radially outer fuel/gas mixer walls 44, 46, and the low momentum of the jets of effusion air 47, the effusion air remains close to the interior surfaces of the fuel/gas mixer walls 44, 46, thus keeping them suitably cool. Despite being efficient and reliable, there can be some issues associated with effusion cooling of the fuel/gas mixer 30.
One is that the effusion air 47 may not mix properly with the fuel injected into the mixing zones 25 of the fuel/gas mixer 30 via the fuel injectors 32, whose outlets are located generally centrally between the radially inner and radially outer walls 44, 46 of each individual mixing zone 25. The effusion air does not, therefore, make much contribution to reducing the flame temperature in the annular combustion chamber 34 and thus to reducing the level of undesirable NOx emissions.
To provide cooling for the fuel injectors 32, to reduce the flame temperature and furthermore to ensure that the fuel emerging from the fuel injectors 32 does not combust prematurely in the presence of the relatively high temperature combustion gases, it may be necessary to provide a supply of carrier air. The carrier air is injected into the mixing zones 25 of the fuel/gas mixer 30 with the fuel, through the fuel injectors 32, and can include re-cooled air from the high pressure compressor 14 but the provision of such carrier air is undesirable and can result in loss of efficiency and power.
There is, therefore, a desire for an improved reheat combustor for a gas turbine engine, and for a reheat combustor with improved cooling which provides for the reduction in flame temperature to reduce the level of undesirable NOx emissions and which also minimizes power and efficiency losses within the gas turbine engine.